1. Field of the Invention
The present invention relates to a radiation imaging apparatus, a processing method therefor, and a radiation imaging system.
2. Description of the Related Art
There are known radiation imaging apparatuses that irradiate a subject with a radiant ray (for example, an X ray) to detect the intensity distribution of the radiant ray, which has penetrated the subject to capture its radiation image. Such radiation imaging apparatuses are provided with a sensor to capture a radiation image.
The sensor includes an insulated substrate such as a glass substrate. For example, a flat-panel type area sensor includes a plurality of image capture elements, each including conversion elements and switch elements (Thin film transistors (TFTs)), two-dimensionally arranged on the insulated substrate.
With a radiation imaging apparatus, radiant ray such as the X ray is converted into electric charges by conversion elements and then the TFTs arranged in matrix form are driven. Thus, an electric signal caused by the electric charges accumulated in the image capture elements is read, and a resultant subject image (radiation image) is acquired.
This radiation image (i.e., the read electric signal) contains an offset component generated in the area sensor and a reading unit. Since the above-described offset component is contained in the image captured through radiant-ray irradiation, to remove the offset component from the acquired image, offset correction is to be performed.
The following two different offset correction methods are known. One offset correction method acquires an offset correction image in advance and performs offset correction by using its data. With this method (first method), an electric signal caused by electric charges accumulated in the image capture elements is read from the area sensor in a state where there is no radiant ray or light based thereon incident on the area sensor. Thus, the offset correction image is acquired.
The offset correction image is stored in a memory. Then, at the time of radiant-ray-based imaging, the offset component is removed from the acquired radiation image by using the offset correction image stored in the memory.
The first offset correction method is beneficial for quickly performing imaging since it is not necessary to acquire an offset correction image at each time of imaging. However, it is commonly known that the offset component in a flat-panel type area sensor may be affected by various factors such as temporal change, temperature change, afterimage (an effect by an optical history of preceding frame), defective pixel change, and so on.
This means that the offset component may change and that the first offset correction method does not provide sufficient offset correction. In some cases, for example, offset correction may contrarily degrade the radiation image quality.
The other offset correction method acquires an offset correction image at each time of radiation imaging. With this method (second method), after radiation imaging, an offset correction image is acquired in a state where there is no radiant ray or light based thereon incident on the area sensor. Then, the offset component is removed from the radiation image by using the offset correction image acquired at each time of radiation imaging. The second offset correction method acquires an offset correction image at each time of imaging and therefore takes a comparatively long time.
Japanese Patent Application Laid-Open No. 2008-036405 discusses a technique for partially driving only a part of image capture elements to shorten the time for acquiring an offset correction image. The technique in Japanese Patent Application Laid-Open No. 2008-036405 performs synthetic processing by using specific factors when generating an offset correction image.
Therefore, offset correction using an offset correction image is largely affected by these factors. If appropriate factors are not set, noise is generated in the offset correction image, which may cause degradation of the radiation image quality. In this way, a radiation imaging apparatus uses techniques for shortening the imaging time and appropriately removing the offset component contained in the radiation image.